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Seeking to fabricate B4C composites that are even more superhard (>30 GPa) at lower cost, B4C was transient liquid-phase assisted spark-plasma-sintered, somewhat counterintuitively, at lower temperature ( 15 vol.%) than ever before. It was found that just 20 vol.% MoSi2 aid enables the full densification of B4C at 1700 °C, thereby avoiding the deleterious transformation β-MoB2→α-MoB2, having consumed the entire MoSi2 to form MoB2 and SiC. This maximizes the hardness (∼33 GPa) of these novel triplex-particulate B4C-SiC-MoB2 composites without penalizing their toughness (∼4.1 MPa⋅m0.5). Also importantly, the dry sliding-wear of these novel composites was investigated for the first time, showing that they undergo only mild wear (specific wear rate of ∼10−8 mm³/(N⋅m)) by plasticity-dominated two-body abrasion. Moreover, it was demonstrated that they are much more wear resistant than porous B4C monolithics fabricated under both the same and more demanding conditions, and at least as equally wear resistant as fully-dense B4C monolithics and composites fabricated under more demanding conditions.

An ultrahard ZrC–Co cemented carbide was fabricated by high-energy ball-milling (HEBM) of micrometric starting powders and spark plasma sintering (SPS), whose ultrafine-grained microstructure makes it comparatively more sliding-wear resistant. First, it is shown that prolonged HEBM activates mechanically the ZrC+Co powder mixture, enhancing markedly its SPS densifiability (notable reduction of the SPS temperature attributable to the lowered eutectic temperature and speeded kinetics of solution-reprecipitation). The combination of refined particle sizes and enhanced sinterability yields ultrafine-grained ZrC–Co cemented carbides with ultrahigh hardness (~18 GPa) and sufficient fracture toughness (~6 MPa·m1/2). And second, it is shown that dry wear of the ZrC–Co cemented carbides is, regardless of the microstructure, mild (10−7 mm3/(N·m)) and occurs essentially by plasticity-dominated abrasion, but also that their microstructural refinement makes them comparatively much more wear resistant and therefore attractive for tribological applications.

The microstructural development during crystallization firing of a commercially-available dental-grade nanostructured lithia-zirconia glass-ceramic (Vita Suprinity® PC) was unraveled using a wide battery of ex-situ and in-situ characterization techniques. It was found that the milling blocks are slightly crystallized glass-ceramics, with a complex chemical composition and consisting of partially de-polymerized glass plus lithium silicate (Li2SiO3) nanocrystals. It was also found that during crystallization firing the glassy matrix first reacts with part of the Li2SiO3 to form lithium disilicate (Li2Si2O5) at ∼810−820 °C, and then lithium orthophosphate (Li3PO4) precipitates from the glass. This results in glass-ceramics with abundant nanocrystals embedded in a sparse zirconosilicate glass matrix (containing many other cations subsumed) that, due to its high viscosity, inhibited crystal growth. Therefore, these dental glass-ceramics are not reinforced with zirconia (ZrO2) crystals unless over-fired above ∼890 °C and at the expense of its singular nanostructure. Finally, this study opens doors for optimizing the clinical performance of these dental glass-ceramics via microstructural tailoring.

Ceramic/polymer dental composites inspired on the microstructure of human enamel are produced by robocasting combined with polymer infiltration. Robocasting is first used to additively manufacture a network of interconnected hydroxyapatite (HAp) rods aligned perpendicular to the occlusal surface, mimicking the natural material. A co-continuous composite is then obtained by immersing the sintered ceramic skeleton in an epoxy resin bath that is subsequently cured. The mechanical performance of the resulting bioinspired composite was evaluated through biaxial bending and sliding-wear tests. The results obtained are compared with those of two commercial dental composites. Despite being a first prototype, the bioinspired material exhibited mechanical properties very close, both in terms of biaxial strength (63 ± 4 MPa) and wear resistance (specific wear rate (7 ± 2)·10−5 mm3/Nm), to the commercial solutions. The proposed ground-breaking strategy appears, then, as a very promising alternative for the production of innovative dental composites with near-net shape and improved durability.

The densifiability by spark plasma sintering (SPS) and the dry sliding-wear behaviour of novel cemented carbides based on micrometre-sized ZrC with 16.7 vol.% Co binder (ZrC–Co), compositionally equivalent to the typical WC–10 wt%Co but with ZrC in place of WC, were investigated. Firstly, detailed analysis of both the densification and the shrinkage-rate curves registered during SPS (at 50 MPa) showed that ~1275 °C is the onset SPS temperature for liquid-phase sintering, and ~1600 °C that of full densification. Hardness measurements by Vickers indentation tests confirmed the gradual densification with increasing SPS temperature up to 1600 °C, a condition yielding an extra-coarse-grained (~7–8 μm grain size), ultrahard (~18.2 ± 0.3 GPa) ZrC–Co cemented carbide that is also sufficiently tough (~6.1 ± 0.2 MPa m1/2). And secondly, pin-on-disk tribological tests (at both 20 N for 1000 m and 40 N for 2000 m) and subsequent characterization of the worn surfaces indicated that, under sliding contact, these ZrC–Co cemented carbides wear essentially by two-body abrasion dominated by plastic deformation. Wear increases with increasing severity of the contact conditions, but is nonetheless mild due to the high hardness of these ZrC–Co cemented carbides and to the formation of discontinuous oxide tribolayers.

In the quest for novel ceramics for tribological applications via bioinspired design, the differences in the fracture modes that arise upon scratching relevant locations of ceramic-like tooth enamel are investigated. It is found that fracture initiates from weak rod-sheath interfaces at relatively low loads, independent of the sliding direction. However, the geometry and propagation of the cracks depends on the orientation of the interfaces relative to the maximum tensile stress: scratching along the occlusal surface propagates approximately sinusoidal cracks, parallel to the sliding direction, while scratching along the cross-section produces straight cracks that propagate normal (scratch parallel to occlusal surface) or parallel (scratch perpendicular to occlusal surface) to the sliding direction. The formation of cracks is hindered in scratching near the enamel-dentine junction. Implications for the microstructural design of triboceramics (bulks and coatings) with improved durability are discussed.

A critical comparison is made between the dry sliding-wear resistance of a B4C composite fabricated by transient liquid-phase sintering with Ti-Al intermetallic additive and two reference monolithic B4C ceramics fabricated by solid-state sintering. It is shown that, as a consequence of its full densification and super-hardness, the B4C composite is, despite containing secondary phases, markedly more wear resistant (significantly lower coefficient of friction, specific wear rate, worn volume, and wear damage) than the reference monolithic B4C ceramic fabricated under identical spark-plasma-sintering (SPS) conditions, and at least as wear resistant as the reference monolithic B4C ceramic fabricated at much higher SPS temperature. In all materials, wear is nonetheless mild and occurred by two-body abrasion dominated by plastic deformation at the micro-contact level plus, in the porous reference monolithic B4C ceramic, three-body abrasion dominated by fracture. Implications for the lower-cost manufacture of superhard B4C tribocomponents are discussed.

A wide suite of powder mixtures of ZrB2 with five different additions of transition metal disilicides (MeSi2; 5, 17.5, or 30 vol% MoSi2; 17.5 vol% TaSi2; or 17.5 vol% ZrSi2) were prepared by high-energy ball-milling (HEBM) for different times, and their optimal densification temperatures were evaluated by dilatometric spark-plasma-sintering (SPS) tests to compare their sinterability. SPS densification tests at the so-determined optimal densification temperatures were also performed in selected cases. It was found that HEBM progressively enhanced the sinterability, especially once the ZrB2 particle sizes were refined to the ultrafine range or below (

Superhard composites of B4C reinforced with randomly-oriented reduced graphene oxide (rGO) nanoplatelets are manufactured by a near-net-shape fabrication route based on three successive steps. Firstly, aqueous colloidal processing is used for the environmentally-friendly preparation of a semi-concentrated multi-component slurry (B4C as main component, Ti-Al as sintering additive, and rGO as toughening reinforcement), whose suitability for wet shaping is demonstrated by rheological measurements. Secondly, slip casting is used to produce robust green parts with shapes on demand and microstructures free of macro- and micro-defects. And thirdly, pressureless spark-plasma sintering (PSPS) is used for the ultrafast and energy-efficient densification of the green parts with shape retention. Measurements of shrinkage and hardness, as well as the microstructural observations, are used to identify suitable PSPS temperatures leading to obtaining isotropic B4C/rGO composites that are superhard and almost twice as tough as the monolithic B4C ceramics.

A fabrication route based on aqueous colloidal processing plus transient liquid-phase assisted spark-plasma-sintering (SPS) with Ti-Al additives is described for the environmentally friendly obtention of superhard B4C composites reinforced with reduced graphene oxide (rGO) having orthotropic and isotropic microstructures. It is shown that the former, which have coarse rGO platelets preferentially aligned perpendicular to the SPS pressing direction, can be prepared from mixtures of B4C and Ti-Al particles with a source of thick, large rGO nanoplatelets by imposing smooth co-dispersion conditions to avoid platelet re-exfoliation and fragmentation. The latter, which have fine rGO platelets randomly oriented, can be fabricated from mixtures of B4C and Ti-Al particles with a source of thin, small rGO nanoplatelets by applying intensive sonication to promote platelet re-exfoliation and fragmentation during co-dispersion. Finally, it is shown that these orthotropic and isotropic B4C/rGO composites are equally superhard, and that, as expected, their microstructures interact differently with the cracks. Finally, this processing route is simple, and easily adaptable/extensible to make other ceramic/rGO composites with orthotropic and isotropic microstructures.

OBJECTIVE. To elucidate the compositional and microstructural developments of a novel lithium silicate glass-ceramic during its crystallization cycle. METHODS. Blocks of a lithium silicate glass-ceramic (Obsidian®, Glidewell Laboratories) were cut into 1 mm thick plates and polished to 1 μm finish. Some of them were crystallized prior to polishing. Firstly, ex-situ compositional and microstructural characterizations of both the pre- and post-crystallized samples were performed by wavelength dispersive X-ray fluorescence, field-emission scanning electron microscopy (FE-SEM), and X-ray diffractometry (XRD). Secondly, the pre-crystallized samples were subjected to in-situ compositional and microstructural characterizations under non-isothermal heating by simultaneous thermogravimetry/differential scanning calorimetry, X-ray thermo-diffractometry, and field-emission scanning electron thermo-microscopy. RESULTS. The microstructure of pre-crystallized Obsidian® consists of an abundant population of perlitic-like/dendritic lithium silicate (Li(2)SiO(3)) nanocrystals in a glass matrix. Upon heating, the residual glassy matrix does not crystallize into any form of SiO(2); elemental oxides do not precipitate unless over-heated above 820°C; and the Li(2)SiO(3) nanocrystals do not react with the glassy matrix to form typical lithium disilicate (Li(2)Si(2)O(5)) crystals. Nonetheless, the Li(2)SiO(3) nanocrystals grow and spheroidize through the solution-reprecipitation process in the softened glass, and new lithium orthophosphate (Li(3)PO(4)) nanocrystals precipitate from the glass matrix. SIGNIFICANCE. The identification of compositional and microstructural developments of Obsidian® indicates that, by controlling the firing conditions, it is possible to tailor its microstructure, which in turn could affect its mechanical and optical properties, and ultimately its clinical performance.

The lubricated sliding-wear resistance of a fine-grained B4C composite fabricated by transient liquid-phase assisted spark-plasma sintering with Ti–Al additives at smooth conditions is critically compared to that of its reference monolithic B4C ceramic. It is shown that the former has an excellent sliding-wear resistance that exceeds that of the later by one order of magnitude (far less specific wear rate, worn volume, and wear damage), attributable to its greater hardness and densification. The wear mode is abrasion dominated by plastic deformation, plus localized microfracture in the monolithic ceramic. Implications for fabricating highly wear-resistant tribocomponents based on superhard ceramics are presented.

The fabrication of B4C ultrafiltration membranes is described. Firstly, a semi-dilute B4C slurry was environmentally-friendly prepared by aqueous colloidal processing, optimizing its dispersion by sonication, and used to deposit B4C membranes onto SiC macro-porous supports by dip-coating. Secondly, the resulting green membranes were characterised microstructurally by scanning electron microscopy (SEM), and pressureless sintered within the intermediate sintering regime. Thirdly, the sintered membranes were calcined in air to clean them from possible free carbon in the smallest pores, with the optimal calcination conditions having been identified by thermogravimetry coupled with mass spectrometry. Next, the calcined, sintered membranes were characterised microstructurally by SEM, tested mechanically against scratching, and characterised texturally by capillary flow porometry, thus identifying the optimal among them. Lastly, as a complement to the fabrication study, the filtration permeability of the optimal membrane was evaluated using deionized water. This work thus paves the way towards the fabrication of ceramic membranes based on B4C, lighter and potentially more durable than others, for filtration applications.

The aqueous colloidal processing of submicrometre B4C powder (∼0.6 μm) with coarse Ti-Al powder (∼40 μm) as sintering additive was investigated. Firstly, by measuring the zeta potential, pHs were identified that promote the individual colloidal stability of the B4C and Ti-Al particles as well as their co-dispersion in water with two different deflocculants (one anionic and the other cationic). It was found that the anionic and cationic deflocculants shift the isoelectric points of B4C and Ti-Al to more acidic and more basic pHs, respectively, making their co-dispersion possible at neutral pH. And secondly, by means of rheological studies, conditions were identified (sonication time, deflocculant type, and deflocculant content) that at quasi-neutral pH yield B4C + Ti-Al shear-thinning concentrated suspensions (30 vol.% total solids) with low viscosity and small hysteresis loop. Interestingly, those deflocculated with the cationic polyelectrolyte had better rheological behaviour, being also less viscous and almost non-thixotropic. These suspensions were freeze-dried, obtaining powder mixtures that were compacted by conventional spark plasma sintering (SPS), and also slip-cast, obtaining robust green pieces that were densified by pressureless SPS. The two B4C composites thus obtained are superhard, with Vickers hardnesses greater than 30 GPa.

Toughened, super-hard B4C triplex-particulate composites were densified by spark plasma sintering with MoSi2 additives (5, 10, and 15 vol.%) at temperatures in the range 1750–1850 °C at which the reference monolithic B4C ceramics are porous. It is proved that MoSi2 is a reactive sintering additive that promotes densification by transient liquid-phase sintering, thus yielding fully-dense B4C-MoB2-SiC composites at relatively lower temperatures. Specifically, the MoSi2 first reacts at moderate temperatures (

An investigation is made of wear mechanisms in a suite of dental materials with a ceramic component and tooth enamel using a laboratory test that simulates clinically observable wear facets. A ball-on-3-specimen wear tester in a tetrahedral configuration with a rotating hard antagonist zirconia sphere is used to produce circular wear scars on polished surfaces of dental materials in artificial saliva. Images of the wear scars enable interpretation of wear mechanisms, and measurements of scar dimensions quantify wear rates. Rates are lowest for zirconia ceramics, highest for lithium disilicate, with feldspathic ceramic and ceramic-polymer composite intermediate. Examination of wear scars reveals surface debris, indicative of a mechanism of material removal at the microstructural level. Microplasticity and microcracking models account for mild and severe wear regions. Wear models are used to evaluate potential longevity for each dental material. It is demonstrated that controlled laboratory testing can identify and quantify wear susceptibility under conditions that reflect the essence of basic occlusal contact. In addition to causing severe material loss, wear damage can lead to premature tooth or prosthetic failure.

The objective of this study is to elucidate the interdependence of competing mechanical degradation processes in biphasic dental materials with ceramic constituents in the region of high-pressure occlusal loading. It is hypothesized that wear resistance in this region correlates inversely with basic material parameters (modulus, hardness, toughness, strength) evaluated from 'standardized' test specimens. Ball-on-flat wear tests in simulation of oral function are used to quantify susceptibility to protracted sliding contact damage. Wear rates for this class of dental material tend to increase with quasistatic parameter values, so the latter do not provide a reliable guide to longevity. The generation of severe-wear facets involves cumulative quasiplastic deformation and microcrack coalescence at the grain level. It is implied that interplay between wear and fracture mechanisms should be an important consideration in future microstructural design of dental ceramics, especially in the quest to balance durability against esthetics.

Composite bioinks made of 45S5 Bioglass ® and β-TCP (β-tricalcium phosphate) were elaborated from raw powders using carboxymethyl cellulose (CMC) as a single multifunctional processing additive. The developed inks were used to fabricate straight rods, which were then sintered at 1000 °C and 1100 °C. The densities and bending strengths of the sintered rods were measured, and both parameters were found to increase with the 45S5 bioglass content, most dramatically beyond 60 vol% of 45S5 bioglass. X-Ray Diffraction analysis revealed the occurrence of significant changes in the phase composition of the material, including formation of new phases like parawollastonite, rhenanite or hydroxyapatite. Comparison of the results obtained for pure TCP with those of previous studies, where other processing additives different from CMC were used, suggests that CMC might be hampering the sintering of β-TCP.

Robocasting and pressureless spark plasma sintering are combined for the first time to fabricate geometrically-complex B4C components. It is shown that robocasting allows B4C green pieces to be printed with near-net shape from inks with suitable rheological properties, and that subsequent pressureless spark plasma sintering permits an ultrafast, energy-efficient, solid-state densification that yields B4C parts with adequate mechanical properties. Furthermore, the usefulness of cold-isostatic pressing to improve the densification of the pieces is evaluated, and the benefits of robocasting over conventional dry powder compaction are identified. Finally, the scalability for the production of large B4C pieces is discussed.

The comminution of a typical submicrometre B4C powder with a high-energy mill (i.e., a shaker mill) operated in air in either a dry or a wet environment was investigated. It was found that dry shaker milling (i.e., high-energy ball-milling) is able to progressively refine the B4C particles to the nanoscale. While this is accompanied by oxidation and aggregation, these are not serious drawbacks. Wet shaker milling in methanol (i.e., conventional ball-milling) resulted only in a moderate B4C particle refinement with greater contamination by the milling tools, which limits its usefulness. It was also found that both dry and wet milling modify the B4C crystal structure, attributable to carbon enrichment. Consequently, dry shaker milling was found to be more recommendable than wet shaker milling to provide B4C starting powders with superior sinterability. A comparative densification study by spark-plasma sintering confirmed this recommendation, and also showed the usefulness of dry shaker milling to obtain refined B4C microstructures for structural applications.

The appropriate conditions for liquid-phase assisted spark-plasma sintering (SPS) were identified for the fabrication of both SiC nanoceramics and their nanocomposites with carbon nanotubes (CNTs). A parametric study of the nanoceramics and nanocomposites with a given type of CNTs showed that the SPS temperature (as measured by the radial optical pyrometer) optimizing their densification, nanograin size, and mechanical properties is 1700 °C (soaking for a few minutes), below which there is incomplete densification, and above which there is obvious grain growth with no benefit in hardness or toughness in the case of the nanoceramics, and prejudicial to both properties in the case of the nanocomposites due to the CNT degradation. It was also shown that the nanocomposites have smaller nanograins than their nanoceramic counterparts, and are softer but tougher. Extension to nanocomposites with different types of CNTs confirmed these trends, and showed that the CNT features do not condition the densification, microstructure or mechanical properties of these nanocomposites.

The damage to human dental enamel under cyclic, axial contacts in a silica particle medium is investigated. It is found that such damage is hierarchical, affecting different length-scales of the enamel structure. At the contact surface, it consists of micron-sized defects, with an attendant increase of surface roughness due to microindentation of the abrasive particles. Below the surface, demineralization of the enamel is observed, which is attributable to inelastic processes at the nanoscale. Axial-only contacts in particulate media result in negligible wear at the macroscopic scale, but may degrade the fracture strength. Potential implications of these results in the fields of dentistry and biology are discussed.

The effect of different dip-coating variables-solvent, deposition temperature and polymer concentration-on the mechanical performance of polycaprolactone-coated 45S5 bioglass robocast scaffolds is systematically analyzed in this work. The reproducible geometry of the scaffolds produced by this additive manufacturing technique makes them an optimal model system and facilitates the analysis. The results suggest that the mechanical performance of the hybrid scaffolds is improved monotonically with polymer concentration, but this concentration cannot be increased indefinitely if the macroporosity interconnectivity, and thus the scaffold׳s capacity to promote tissue ingrowth, are to be preserved. An optimal concentration, and therefore viscosity (~1-4Pas in the present case), exists for any given set of process variables (scaffold geometry and material, polymer, solvent and process temperature) that yields coatings with optimal reinforcement and minimal reduction of scaffold functionality. Solvent and process temperature do not directly affect the strengthening provided by the polymeric coating. However they can determine the maximum concentration at the critical viscosity, and thereby the maximum achievable mechanical performance of the resulting hybrid scaffold.

The effects were individually investigated of ZrC grain size and MoSi 2 content on the lubricated sliding-wear resistance of ZrC–MoSi 2 triboceramics densified by spark-plasma sintering (SPS). Microstructures with different average grain sizes were obtained by varying the high-energy ball-milling time of the ZrC–MoSi 2 powder mixtures and their SPS temperature, while the microstructures with different secondary phase contents were obtained by varying the relative concentrations of ZrC and MoSi 2 in the powder batches. Experimentally, it was found that the ZrC–MoSi 2 triboceramics are all highly wear resistant, but also that wear resistance decreases as the ZrC grain size and MoSi 2 content increase. The dominant wear mode was identified as abrasion, and the prevailing wear mechanism was plastic deformation accompanied by localized fracture. Accordingly, the wear trends are explained in terms of the hardness of the ZrC–MoSi 2 triboceramics. Finally, the implications are discussed of providing the electrically-conductive ZrC–MoSi 2 triboceramics with greater wear resistance via controlled processing.

SiC nanostructured ceramics without and with carbon nanotubes (CNTs) were prepared by aqueous colloidal processing plus liquid-phase assisted spark-plasma sintering, and their sliding wear behaviour compared. It was found that the SiC nanoceramic is highly resistant to wear, with specific wear rates in the order of 10−8 mm3/N m or lower under the tribological conditions used, but also that the SiC/CNT nanocomposite is even more so. Qualitatively, the nanocomposite displayed a simple wear curve with a steady-state wear rate, over the wear curve of the nanoceramic with two wear rates, the second being one order of magnitude greater than the first. Quantitatively, the nanocomposite wore less overall with smaller worn volume, and especially had a lower wear rate (particularly marked for moderate and prolonged sliding distances). It was also found that the superior wear resistance of the SiC/CNT nanocomposite is due to combined effects of lubrication and toughening provided by the CNTs, the former reducing the severity of the sliding-contact conditions and the latter hindering the propagation and coalescence of the short cracks responsible for nanograin pull-out. CNT addition thus emerges as a hopeful line for processing ceramic nanocomposites that are both toughened and highly wear resistant for use in contact-mechanical and tribological applications.

The spark plasma sinterability and the dry sliding-wear resistance of ultrafine-grained WC densified with ~16.7 vol% of Co, Co+Ni, and Co+Ni+Cr, compositions equivalent to that of the typical WC-10wt.%Co, were investigated and compared critically. Firstly, it was found that the partial substitutions of Co by Ni or by Ni+Cr are detrimental for the pressureless ultrafast sinterability, but much more in the latter than the former case, attributable to the higher eutectic temperatures during liquid-phase sintering. However, it was also observed that, thanks to the auxiliary stress supplementing the sintering stresses, the ultrafast sinterability with pressure is affected by these partial substitutions either not at all (Co by Ni) or very little (Co by Ni+Cr), making it possible in all cases to obtain fully-dense WC cermets with almost no grain growth at the same spark plasma sintering temperature. And secondly, it was found that these cemented carbides are very resistant to dry sliding wear, but that WC-(Co+Ni+Cr) is markedly more so, attributable essentially to its greater hardness. This is due to wear occurring essentially by two-body abrasion dominated by plastic deformation (ploughing), plus some oxidative wear with formation of tribo-oxidation layers that, although not entirely coherent, are nonetheless beneficial for the wear resistance.

OBJECTIVE. To elucidate the microstructural evolution of a commercial dental-grade lithium disilicate glass-ceramic using a wide battery of in-situ and ex-situ characterization techniques. METHODS. In-situ X-ray thermo-diffractometry experiments were conducted on a commercially available dental-grade lithium disilicate glass-ceramic under both non-isothermal and isothermal heat treatments in air. These analyses were complemented by experiments of ex-situ X-ray diffractometry, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, differential scanning calorimetry, and field-emission scanning electron thermomicroscopy. RESULTS. It was found that the non-fired blue block consists of ~40 vol.% crystals embedded in a glass matrix. The crystals are mainly lithium metasilicate (Li(2)SiO(3)) along with small amounts of lithium orthophosphate (Li(3)PO(4)) and lithium disilicate (Li(2)Si(2)O(5)). Upon heating, the glassy matrix in the as-received block first crystallizes partially as SiO(2) (i.e., cristobalite) at ~660 °C. Then, the SiO(2) crystals react with the original Li(2)SiO(3) crystals at ~735 °C, forming the desired Li(2)Si(2)O(5) crystals by a solid-state reaction in equimolar concentration (SiO(2) + Li(2)SiO(3) → Li(2)Si(2)O(5)). Precipitation of added colourant Ce ions in the form of CeO(2) appears at ~775 °C. These events result in a glass-ceramic material with the aesthetic quality and mechanical integrity required for dental restorations. It also has a microstructure consisting essentially of elongated Li(2)Si(2)O(5) grains in a glassy matrix plus small cubic CeO(2) grains at the outermost part of the surface. SIGNIFICANCE. It was found that by judiciously controlling the heat treatment parameters, it is possible to tailor the microstructure of the resulting glass-ceramics and thus optimizing their performance and lifespan as dental restorations.

The effect of polymer infiltration on the mechanical properties of 13–93 bioactive glass scaffolds fabricated by robocasting has been investigated. Biodegradable polymers—poly (lactic acid) (PLA) and poly (e-caprolactone) (PCL)—were infiltrated into selected robocast scaffolds by immersion of the structure in a polymer melt. The mechanical enhancement achieved upon impregnation on robocast 13-93 structures with fully-dense struts was found to be lower than in the currently existing literature due to the absence of microporosity. The modulus increase is negligible and the strengthening achieved is limited (

The conditions under which ZrC triboceramics are fabricated (high-energy ball-milling and spark-plasma sintering (SPS)) lead to major microstructural differences. The effects of these conditions on the resulting ceramic׳s lubricated sliding-wear resistance were investigated. One condition studied entailed SPS of the as-purchased micrometre powder, giving super-coarse-grained ceramics with little residual porosity. Another was SPS at the same temperature for a shorter time, with the powder having been subjected to short high-energy ball-milling, yielding near-dense, coarse-grained ceramics. The last was with the powder subjected to prolonged high-energy ball-milling, followed by SPS in smother conditions in terms of temperature and time, resulting in fully-dense, fine-grained ceramics. It was found that, while all three were highly wear resistant, the ceramic obtained by combining prolonged ball-milling with smooth SPS was the most so, with less damage and a lower steady-state wear rate coefficient, ascribable to the greater hardness and toughness deriving from the microstructural refinement and densification. The results therefore demonstrate the importance of the processing conditions in obtaining electrically-conductive ZrC triboceramics with superior wear resistance.

The effect of sintering temperature on the mechanical performance of 45S5 Bioglass ® scaffolds fabricated by robocasting and infiltrated by ϵ-polycaprolactone (ϵ-PCL) has been investigated in this work. Infiltration was made by immersion of the robocast scaffolds in a polymer melt to produce fully impregnated, dense co-continuous composites, or in a polymeric solution to produce a polymer-coated bioceramic structure. Polymer infiltration produced a significant increase in the strength and toughness of the scaffolds, both under compression and bending. The strengthening and toughening provided by the polymer was larger in fully impregnated structures than in coated scaffolds, and the toughening was especially dramatic (up to more than two orders of magnitude) under flexural stresses. The strengthening and toughening produced by polymer infiltration increased monotonically with the level of microporosity in the ceramic struts, i.e., they decreased with the sintering temperature. In spite of that, denser ceramic struts yield stronger and tougher hybrid structures after polymer infiltration.

The elasto-plastic properties and contact damage evolution of ZrB 2 ultra-high-temperature ceramics (UHTCs) pressureless sintered at 1900 °C with MoSi 2 , TaSi 2 , or ZrSi 2 additive were evaluated by Hertzian indentation tests in air up to 800 °C. It was found that liquid-phase sintering occurred in the three cases, but with severe decomposition of the TaSi 2 and ZrSi 2 additives and extensive formation of (Zr 1− m Si m )B 2 solid-solutions in the case of the ZrSi 2 additive. Furthermore, ZrB 2 sintered with ZrSi 2 underwent severe peptization at 800 °C, raising questions as to its utility as a UHTC. It was also found that ZrB 2 sintered with MoSi 2 exhibits the best combination of contact-mechanical properties, being harder, stiffer, and more resistant to quasiplastic damage and radial cracking than ZrB 2 sintered with TaSi 2 or ZrSi 2 , as well as equally resistant to ring/cone cracking from ∼600 °C onwards. Implications of interest for the UHTC community are discussed.

The sliding-wear resistance of pure near fully-dense B4C is investigated, and the wear mode/mechanisms identified, under lubrication with water, diesel fuel, and paraffin oil. It is found that the wear is mild in the three cases, with specific wear rates (SWRs) of 10−16–10−17 m3/N m. Nonetheless, the wear resistance of the B4C ceramic is one order of magnitude greater under oil lubrication (1016 N m/m3) than under water lubrication (1015 N m/m3), and twice as great for the specific case of paraffin oil than diesel fuel, attributable to the lubricant’s viscosity. It is also found that the wear mode is always abrasion, and that the wear mechanisms are plastic deformation and localized fracture with grain pullout. However, in agreement with the macro-wear data, the severity of the wear damage is lower under lubrication with paraffin oil, followed by diesel fuel, and lastly water. Finally, microstructural considerations are discussed with a view to enhancing the sliding-wear resistance of B4C triboceramics.

A route based on aqueous colloidal processing followed by liquid-phase assisted spark-plasma-sintering (SPS) is described for fabricating eco-friendly nanocomposites of SiC with nano-carbonaceous phases (nanotubes, nanoplatelets, or nanoparticles). To this end, the conditions optimizing the aqueous colloidal co-dispersion of SiC nanoparticles, Y3Al5O12 nanoparticles (acting as sintering additives), and carbon nanotubes (CNTs), graphene oxide (GO) nanoplatelets, or carbon black (CB) nanoparticles were first identified. Next, homogeneous powder mixtures were prepared by freeze-drying, and densified by liquid-phase assisted SPS, thus obtaining nanocomposites of SiC with CNTs, reduced GO (rGO) nanoplatelets, or pyrolized + graphitized CB (p + gCB) nanoparticles. It is also shown that these nanocomposites are dense and have a high hardness of ∼20 GPa regardless of the nano-carbonaceous phase chosen, but are markedly tougher with CNTs and rGO (i.e., with high aspect ratio nano-carbonaceous phases). Finally, arguments are provided for the appropriate choice of nano-carbonaceous phases for engineering ceramic nanocomposites.

The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.

The effect of a 2 wt% addition of particulate graphite on the spark-plasma sintering (SPS) kinetics of both ZrB2 and ZrB2+30 vol% SiC was investigated using to that end two broad sets of powder mixtures prepared by high-energy co-ball-milling. It was observed that the particulate graphite addition enhances the SPS kinetics of both systems, and it was identified that, contrary to the case in conventional sintering, this improvement in sinterability is not due to the carbothermal reduction of oxides in situ during SPS but to the lubrication imposed by the graphite flakes inducing a greater green-body densification. Consequently, the graphite is not consumed during SPS but remains in the microstructure of the resulting ultra-high-temperature ceramics (UHTCs) as flaky particles dispersed homogeneously at grain boundaries. Implications of interest for the UHTC community are discussed.

The influence of polymer infiltration on the flexural strength and toughness of β-tricalcium phosphate (β-TCP) scaffolds fabricated by robocasting (direct-write assembly) is analyzed. Porous structures consisting of a tetragonal three-dimensional lattice of interpenetrating rods were impregnated with biodegradable polymers (poly(lactic acid) (PLA) and poly(e-caprolactone) (PCL)) by immersion of the structure in a polymer melt. Infiltration increased the flexural strength of these model scaffolds by a factor of 5 (PCL) or 22 (PLA), an enhancement considerably greater than that reported for compression strength of analogue materials. The greater strength improvement in bending was attributed to a more effective transfer of stress to the polymer under this solicitation since the degree of strengthening associated to the sealing of precursor flaws in the ceramic rod surfaces should remain unaltered. Impregnation with either polymer also improved further than in compression the fracture energy of the scaffolds (by more than two orders of magnitude). This increase is associated to the extraordinary strengthening provided by impregnation and to a crack bridging toughening mechanism produced by polymer fibrils.

The effect of flaky hexagonal (h) BN additions (1, 5, and 10 vol.%) on the lubricated sliding-wear behavior of fine-grained, liquid-phase-sintered SiC ceramics fabricated by spark-plasma sintering was investigated. It was found that resistance to the initial mild, deformation-controlled wear decreases with increasing h-BN content in the composite, which progressively exhibits a greater wear rate and a sooner transition to severe wear. This is because the softer h-BN particles reduce the hardness and do not act as internal lubricant, while promoting poorer grain cohesion due to their morphologically-favored segregation at grain boundaries. By contrast, their addition is increasingly beneficial in terms of resistance to the subsequent severe, fracture-controlled wear upon prolonged sliding contact, with a lower wear rate. This is because the flaky h-BN particles increase the fracture toughness, and also act efficiently as external lubricant when pulled-out from the microstructure. Finally, implications for the design of advanced triboceramics are discussed.

The effect of SiC addition (5, 17.5, or 30 vol%) on the spark-plasma sinterability of ZrC ultra-high-temperature ceramics (UHTC) was investigated as a function of the intensity of high-energy co-ball-milling. It was found that the spark-plasma-sintering (SPS) kinetics of the ZrC–SiC powder mixtures is enhanced progressively with increasing co-milling time, which is due to the progressive refinement of the crystallite sizes. It was also found that in practice the SiC addition is beneficial for the SPS kinetics of ZrC if the co-milling time is short enough so as to refine the SiC crystallite size only to the submicrometre range, although the improvement in sinterability does not correlate with the SiC content. On the contrary, the SiC addition is increasingly detrimental for those co-milling times that promote SiC refinement to the nanoscale. This unexpected trend is due to slower ZrC crystallite size refinement in the presence of SiC and, especially, to the complex role played during SPS by the passivating SiO 2 films formed on the SiC particles, and is different from what has been observed in ZrB 2 –SiC. Finally, implications of interest for the UHTC community are discussed.

Aqueous colloidal processing was used for the environmentally friendly preparation of well-dispersed concentrated suspensions and powder mixtures of submicrometric SiC powders with submicrometric Y3Al5O12 as sintering additive plus diamond nanoparticles as reinforcing phase. It is shown that the addition of nano-diamond markedly increases the viscosity and thixotropy of the SiC + Y3Al5O12 suspensions, and also that, by adjusting the pH, deflocculant content, and sonication time it is possible to co-disperse these three rheologically different ceramic phases (i.e., non-oxide, oxide, and hydrophobic compounds) in aqueous media, thereby avoiding the otherwise irremediable severe hetero-aggregation. Moreover, the microstructural characterization of the powder mixtures obtained by freeze-drying the suspensions confirmed the homogeneous dispersion of the diamond nanoparticles among the submicrometric SiC and Y3Al5O12 particles in the form of isolated or adhered nanoclusters and nanodeposits. Implications for engineering the microstructure of non-oxide ceramics with diamond nanodispersoids are discussed.

The sliding-wear behavior of ultrafine-grained SiC densified by spark plasma sintering (SPS) with 3Y2O3 + 5Al2O3 or Y3Al5O12 additives was compared to elucidate the effect of the intergranular-phase source. Significantly greater sliding-wear resistance (i.e. lower mild- and severe-wear rates, and delayed mild-to-severe wear transition) is observed with the Y3Al5O12 additive, due to its carbothermal reduction during SPS resulting in a harder YAlO3 phase while reducing the intergranular phase content. Implications for the processing of superior SiC triboceramics are discussed.